1. Field of the Invention
The present invention relates to optical metrology, and more particularly to improving the optical metrology process using a pre-aligned metrology system that includes pre-aligned metrology assemblies and pre-aligned metrology modules.
2. Description of the Related Art
In the manufacture of integrated circuits, very thin lines or holes down to 10 nm or sometimes smaller are patterned into photoresist and then transferred using an etching process into a layer of material below on a silicon wafer. It is extremely important to inspect and control the width and profile (also known as critical dimensions or CDs) of these lines or holes. Traditionally the inspection of CDs that are smaller than the wavelength of visible light has been done using large and expensive scanning electron microscopes (SEM). As the structures patterned get smaller and smaller, the measurement precision and accuracy becomes much higher, and additional measurement data is needed for each wafer for process control. It is a very challenging for SEM to meet the metrology request in these cases. In many cases, manufacturers need to measure CD and profiles immediately after the photoresist has been patterned, a non-destructive metrology is needed to overcome photo-resist damage issues by SEM. For the case of process control or advanced equipment control, the measurement has to be non-destructive and the measurement tool need to be integrated into the process tools, such as wafer track that develops the photoresist or wafer-etching tool.
One general technique that has promise for integrated CD measurements is Scatterometry using Optical Digital Profilometry (ODP). Exemplary Scatterometry techniques are described in U.S. Pat. No. 6,538,731, entitled “System and Method for Characterizing Macro-Grating Test Patterns in Advanced Lithography and Etch Processes”, by Niu, et al., issued on Mar. 25, 2003, and is incorporated in its entirety herein by reference. Exemplary ODP techniques are described in U.S. Pat. No. 6,433,878, entitled “Method and Apparatus for the Determination of Mask Rules Using Scatterometry”, by Niu, et al., issued on Apr. 13, 2002, and is incorporated in its entirety herein by reference. This technique takes advantage of the fact that small periodic lines or holes diffract an incident light beam, and the properties of the light in each of the diffraction orders carries information of the lines and holes. In practice, the optical properties of zero-th diffraction order that is reflected (or, for transparent samples, transmitted) from the periodic structures are measured with an optical metrology sensor, and measured data is analyzed with as Scatterometry software, such as ODP. Often such parameters are measured versus wavelength, and in some cases, versus angle of incidence on the sample.
Optical metrology sensor measures the optical properties of the features on a wafer. These optical properties include a fraction of the incident energy reflected and polarization state change. These techniques are described in U.S. Pat. No. 7,064,829, entitled “Generic Interface for an Optical Metrology System”, by Li, et al., issued on Jun. 20, 2006, and is incorporated in its entirety herein by reference. The optical metrology sensor can be designed to sense one or more of this optical properties. For example, a tool that measures energy changes is called a reflectometer, and tools that measure the polarization change are called ellipsometers. The optical metrology sensor typically uses photometric or spectral photometric detectors. For reflectometer, a standard reflector that the reflection is known is needed to deduct the fraction of energy reflected from the feature under test. For ellipsometer, the polarization state of the incident beam is known and stable by design and calibration, and thus standards are not required when the reflectivity is not of interest. The polarization state changes are deduced from the ratios of different Fourier components, thus the absolute light source intensity is not needed. This is often referred to as “self reference”. In either case, the measurement quantity is the optical properties of the feature, and not the intensity of light in the diffraction orders although the optical properties is calculated from the intensities of the diffracted light and standard reflector in reflectometer case. The CD and profile information is obtained from an analysis of the diffraction signal using techniques such regression, library based systems, and machine learning systems such as those based on neural net techniques.
An optical metrology sensor involves directing an incident beam in one or most polarization state at a feature on a wafer, measuring the resulting diffraction signals, and measuring the signal from standard reflector in reflectometer case, the measured signs are first analyzed to find the optical properties of the feature, namely reflectivity or polarization state changes. The measured optical properties of the feature are analyzed to determine various characteristics of the feature. In semiconductor manufacturing, optical metrology is typically used for quality assurance, process control, and equipment control. For example, after fabricating a periodic grating in proximity to a semiconductor chip on a semiconductor wafer, an optical metrology system is used to determine the profile of the periodic grating. By determining the profile of the periodic grating, the quality of the fabrication process utilized to form the periodic grating, and by extension the semiconductor chip proximate the periodic grating, can be evaluated. Further more, the measured dimensions of features can be used to control the process equipment work conditions.
An integrated CD measurement tool must be both fast and compact, and must be non-destructive to the wafer under test. The wafer may also be loaded into the measurement tool at an arbitrary angle creating further complications for instruments that have a preferred measurement orientation with respect to certain wafer features.
Most of the optical metrology systems for CD measurement are stand-alone tools that are used as off-line application for monitoring the process. As the semiconductor roadmap goes to smaller and smaller nodes, more and more challenges on semiconductor process control to meet very tight tolerance while the structure becomes smaller. Integrated metrology tools are needed to measure the structures made on the wafer, and use the measured data for optimizing the process tools that the structures on the wafer has been made, or for adjusting the process tool conditions that the wafer is going to be further processed. For an integrated metrology tools, the reliability and availability need to be much higher than for off line tools, and the maintenance time needs to be significantly shorter than an off line tool.